How to get the quantity of a given weekday in a particular month? For example, get the quantity of Sundays in Jan 2023 or get the quantity of Tuesday in Feb 2023

>Solution :

function getWeekdayCount(year, month, weekday) {
  const firstDay = new Date(year, month, 1);
  const lastDay = new Date(year, month + 1, 0);
  const count = Math.floor((lastDay.getDate() - firstDay.getDate() + (firstDay.getDay() + 7 - weekday) % 7) / 7);
  
  return count;
}

// Example usage:
const year = 2023;
const month = 0; // 0 = January, 1 = February, ..., 11 = December
const weekday = 0; // 0 = Sunday, 1 = Monday, ..., 6 = Saturday

const count = getWeekdayCount(year, month, weekday);
console.log(count); // Output: Number of occurrences of the weekday in the specified month

The formula (lastDay.getDate() - firstDay.getDate() + (firstDay.getDay() + 7 - weekday) % 7) / 7 calculates the number of occurrences of the specified weekday in the month. It subtracts the day of the week of the first day from the day of the week of the last day, adds 7, and takes the modulo 7 to ensure a positive result. Finally, it divides the result by 7 to get the count of occurrences.